1. Field of the Invention
The invention relates generally to integrated circuit packages and manufacturing methods thereof; and more particularly to a kind of integrated circuit package which conducts heat by using an array of carbon nanotubes, and a manufacturing method thereof.
2. Description of Prior Art
Electronic components such as semiconductor chips are becoming progressively smaller along with the development of semiconductor integrated circuits. The dimensions of electronic packages incorporating such chips are also becoming progressively smaller. Nevertheless, the improved operational performance of these chips means that they operate at unprecedented high temperatures. If the increased heat cannot be effectively dissipated, the integrated circuit may malfunction and may even be damaged. Heat dissipation techniques have become increasingly more important along with the development of semiconductor package technology.
A heat conduction coefficient of the material of a conventional semiconductor package is nowadays often considered too low to satisfactorily dissipate heat produced by the operation of a modern, high-speed integrated circuit chip. The use of low heat conduction coefficient material undesirably increases the temperature of the chip, and ultimately results in failure of the chip. One means to solve this problem is shown in FIG. 5. A package comprises a substrate 3, a die 2, several gold wires 4, and an integrated heat spreader 5. The integrated heat spreader 5 is set over the die 2 and adhered on the substrate 3. The die 2, the gold wires 4 and the integrated heat spreader 5 are packaged by plastic potting material 7. A top side of the integrated heat spreader 5 is exposed to ambient air, to dissipate heat produced by operation of the die 2. Alternatively, a heat dissipation device such as a heat sink can be attached to the top side of the integrated heat spreader 5. However, the heat conducted from the die 2 to the integrated heat spreader 5 must pass through the plastic potting material 7, and the plastic potting material 7 has a low heat conduction coefficient. Therefore, the heat dissipation efficacy of the package is poor.
The discovery of thermal interface materials has lead to their incorporation into the ongoing development of heat dissipation techniques in semiconductor package technology. Referring to FIG. 6, this shows a modification of the means described above in relation to FIG. 5. A thermal interface material 6 is set in thermal contact between a top face of the die 2 and an inner face of the integrated heat spreader 5. When the integrated circuit is working, the heat produced is conducted through the thermal interface material 6 to the integrated heat spreader 5, and is then dissipated to the ambient air. However, the effectiveness of the package is still limited by the heat conduction capability of the thermal interface material.
An article entitled “Unusually High Thermal Conductivity of Carbon Nanotubes” and authored by Savas Berber (page 4613, Vol. 84, Physical Review Letters 2000) discloses that a heat conduction coefficient of a carbon nanotube can be 6600 W/mK (watts/milliKelvin) at room temperature. How to apply carbon nanotubes in thermal interface materials for heat dissipation has become an important new field of research.
U.S. Pat. No. 6,407,922 discloses a kind of thermal interface material using carbon nanotubes. The thermal interface material is formed by injection molding, and has a plurality of carbon nanotubes incorporated in a matrix material. A first surface of the thermal interface material engages with an electronic device, and a second surface of the thermal interface material engages with a heat sink. The second surface has a larger area than the first surface, so that heat can be uniformly spread over the larger second surface.
However, the thermal interface material formed by injection molding is relatively thick. This increases a bulk of the thermal interface material and reduces its flexibility. Furthermore, the carbon nanotubes are disposed in the matrix material randomly and multidirectionally. This means that heat does not spread uniformly through the thermal interface material. In addition, heat does not necessarily spread directly from the first surface of the thermal interface material engaged with the electronic device to the second surface of the thermal interface material engaged with the heat sink.
U.S. Pat. Pub. No. 2004/0005736 discloses another thermal interface material comprising an array of carbon nanotubes, and a semiconductor package using a layer of the thermal interface material. The thermal interface material layer is set between a semiconductor chip and a thermal management aid. The thermal management aid is an integrated heat spreader or a heat sink. The array of carbon nanotubes is encapsulated in a matrix of interstitial material. A substantial portion of the nanotubes have a length slightly exceeding the thickness of the interstitial material, to allow the nanotubes to be wedged between the chip and the thermal management aid. This enables heat to be conducted from the chip primarily through the nanotubes, rather than through the surrounding interstitial material.
However, the thermal interface material formed by encapsulating an array of carbon nanotubes in a matrix of interstitial material cannot sufficiently use the heat conduction capability of carbon nanotubes, because of the low heat conduction coefficient of the matrix. In addition, asymmetric radiation of heat from the chip can result in asymmetric heat conduction, and further reduce the efficiency of the thermal interface material. Furthermore, the array of carbon nanotubes needs to be formed on the chip prior to the fabrication of active circuits on the chip, in order to avoid exposing active chip elements to the high temperatures involved in the process of forming the nanotubes. This makes the fabrication of active circuits on the chip problematic.
An integrated circuit package which overcomes the above-mentioned problems and a method for manufacturing such package are desired.